An opto-isolator is a device that transfers a signal optically between two electrical circuits while, at the same time, electromagnetically isolating the circuits from each other. Opto-isolators are used to protect circuits against damage from voltage surges. A transmitter circuit on the transmitter side of the opto-isolator comprises an electrical-to-optical converter, such as a visible or infrared light emitting diode (LED), that converts the electrical signal into an optical signal. A receiver circuit on the receiver side of the opto-isolator comprises an optical-to-electrical converter, such as a photodiode, that converts the optical signal back into an electrical signal.
The transmitter and receiver circuits of an opto-isolator are typically integrated circuits (ICs). It is desirable to integrate these ICs within the same IC package in order to keep the overall size of the opto-isolator small. However, the close proximity of the transmitter and receiver circuits results in capacitive coupling between the ground reference of the receiver IC and the leads that drive the transmitter LED. This capacitive coupling can cause the common mode pulses between the ground reference points of the two circuits to either increase or decrease the drive current on the LED leads. This increased or decreased drive current can affect the On and Off states of the LED, and consequently, the performance of the opto-isolator.
FIG. 1 illustrates a block diagram of a typical opto-isolator 2 having a transmitter IC 3 and a receiver IC 4. The transmitter IC 3 includes an LED control circuit 5 having input interface logic (not shown) for receiving an electrical input signal, and an LED driver circuit (not shown) for generating a drive current that drives an LED 6. The LED 6 is typically separate from the transmitter IC 3 and is usually made using a III-V process technology. The LED 6 is connected by wire bonds (not shown) to the transmitter IC 3. A supply voltage VDD1 and a ground reference GND1 are provided to the transmitter IC 3. The transmitter IC 3 includes a current source 7 for turning the LED 6 on and a shorting switch 8 for ensuring that the LED 6 is turned off when it is supposed to be in the Off state. The transmitter IC 3 also includes an input logic interface (not shown). There is a small, but significant, stray parasitic capacitance between the bond wires going to the LED 6 and the ground reference, GND2, node of the receiver IC 4. This parasitic capacitance is represented by capacitor 9.
The receiver IC 4 includes a silicon photodiode 11, a trans-impedance amplifier (TIA) 12, a comparator 13 and an output driver 14. The optical output of the LED 6 is coupled to the photodiode 11 on the receiver IC 4. The photon input to the photodiode 11 produces a corresponding photo current in the diode 11. This current is amplified in the TIA 12 and then the output is sent to the comparator 13. The comparator 13 compares the output from the TIA 12 to a reference voltage, VTH1, to determine whether the output corresponds to a logic 0 or logic 1 state and provides an output signal to the output driver 14, which produces the output drive signal for the opto-isolator 2 at node 15.
Typically, the operations of the TIA 12, the comparator 13, and the output driver 14 result in a logic 0 being output from the opto-isolator 2 at node 15 if the LED 6 is turned on and the receiver photo current is above the threshold level VTH1. A logic 1 will occur if the LED 6 is turned off. This works well if there is not a significant interfering signal between the transmitter IC 3 and the receiver IC 4. A common mode interference is defined as a signal between the GND1 and GND2 reference points. A key function of the opto-isolator 2 is to permit the transfer of logic signals between two different electrical systems that may be operating at substantially different voltage potentials. This key function is performed well as long as there is not an excessive transient component between the two ground reference points. An excessive transient component is a signal that will disrupt the operation of the isolator.
If the slope of the waveform representing the common mode pulse between the GND1 and GND2 references has a slope greater than about 10 KV/μsec, there will be significant current pulled from the bond wires going to the LED 6 through the parasitic capacitor 9. The relationship between this slope, the parasitic capacitance and the current pulled away from the LED 6 is expressed as:I_error=Cparasitic*dV/dT, where I_error represents the portion of the drive current pulled away from the LED 6 by the parasitic capacitor 9, Cparasitic represents the parasitic capacitance, and dV/dT represents the slope of the common mode pulse. Using this equation, it can be determined that for a common mode pulse having a slope of 10 KV/μsec, the current I_error through a typical Cparasitic value of 50 femptofarads (fF) is 0.5 milliamperes (mA). This current level is relatively high, which means that a significant portion of the drive current for the LED 6 has been pulled through the parasitic capacitance and thereby diverted from the LED 6. This reduces the optical output of the LED 6 and may cause the corresponding signal output from the TIA 12 to drop below the threshold level of the comparator 13, resulting in errors occurring during the operation of the opto-isolator 2.
During an experiment, it was observed that for a common mode signal between GND1 and GND 2 having a slope of 30 KV/usec and Cparasitic=100 fF, the reduction of the LED drive current due to losses through Cparasitic causes the optical output of the LED 6 to be reduced to the point that errors occurred. The reduction in the drive current caused the electrical output from the photodetector 11 to be reduced, which, in turn, caused the voltage signal output from the TIA 12 to drop below the threshold voltage VTH1 of the comparator 13. When this happened, a single LED On pulse received by the receiver 4 resulted in two output pulses at the output node 15 of the opto-isolator 2, which is an improper result.
The traditional approach used to correct this problem is to decrease the size of the parasitic capacitance between the bond wires to the LED 6 and GND2. This can help, but as the IC package geometries become smaller, the dimensions between elements with potential for parasitic capacitances make this adjustment more difficult to achieve. Another approach used to correct this problem is to increase the LED drive current to the point that the perturbations in the drive current caused by the occurrence of common mode pulses between GND1 and GND2 no longer affect the On state of the LED. The use of increased LED drive current, however, also increases the power consumption of the opto-isolator, which is in direct conflict with the dual goals of providing low-power operation in opto-isolators and adequate isolation in various technological applications.
Accordingly, a need exists for a way to correct problems in opto-isolators that are caused by the effects of common mode pulses between the ground references GND1 and GND2 of the transmitter and receiver ICs 3 and 4 and the parasitic capacitance between the wire bonds to the LED 6 and GND2.